I. Field of the Invention
The present invention relates to an overlock sewing machine having a looping width adjusting mechanism and, more particularly, to an overlock sewing machine of the type having a movable claw for adjusting the looping width and carried by a cutter blade holder together with the lower cutter blade of a cutter blade device, the cutter blade holder being adjustably mounted on the machine body. The machine further has a dial and a means for converting the rotation of the dial into the forward and backward movement of the cutter blade holder, wherein the movable claw is movable between an operative position and an inoperative position so that the machine can also perform so-called wrap-lock overlock sewing.
II. Description of the Prior Art
Overlock sewing machines are known which have a stationary claw for forming loops for overlock sewing, a movable claw arranged in a side-by-side fasion to one side of the stationary claw and movable toward and away from the aforesaid side, conversion means for converting operational rotary motion into linear motion for moving the movable claw, and a cutter blade mechanism with a lower cutter blade. This type of overlock sewing machine is shown, for example, in Japanese Laid-Open Application No. 78991/1980. This known overlock sewing machine, however, suffers from the following disadvantage. Namely, since the movable claw is secured to a thin needle plate, the movable claw tends to be oscillate in the vertical direction, i.e. in the direction of the cutter blade operation, due to the influence by cutter blade operation load during the sewing operation of the machine. In consequence, the loop diameter which is determined by the distance between the movable claw and the stationary claw fluctuates to incur a disorder in the seam to impair the appearance of the seam. The conventional overlock sewing machine, therefore, requires means for fixing the movable claw securely to the throat plate by means of, for example, bolts, besides the above-mentioned conversion means for converting the rotary motion into linear motion. Thus, in the conventional overlock sewing machine, it is necessary to mount, the conversion means for converting rotary motion into linear motion and fixing means for fixing the movable claw necessary to adjust looping width, on a thin and narrow throat plate. Such mounting is extremely difficult from requires two-step adjusting and fixing operation. Furthermore, in the conventional overlock sewing machine having the described construction, it is necessary to take off the cloth from the fixing means at each time the looping width is changed. In addition, this type of conventional overlock sewing machine can not perform a special sewing called wrap-lock sewing in which the looping is made in such a manner as to wrap the end of the cloth.
In order to cope with the demand for the wrap-lock sewing, an overlock sewing machine has been proposed in which the movable claw takes either an operative position where it is disposed in a side-by-side fashion to one side of the stationary claw and an inoperative position where it is offset from the stationary claw. In order to mount a means for moving the movable claw between the operative position and the inoperative position and to minimize the influence of the vibration, a block type cutter blade holder is mounted on the machine body by means of a guide so as to be able to move towards and away from the side of the stationary claw, and the movable claw is secured to this block type cutter blade holder. This type of overlock sewing machine is disclosed in, for example, Japanese Patent Publication No. 25875/1980.
In this sewing machine also, the adjustment is made by fingers nipping the cutter blade holder itself, and it is still necessary to fix the above-mentioned guide by a fixing means. Thus, this type of sewing machine still requires necessity for taking two independent steps of adjustment and fixing.
Japanese Laid-Open Application No. 146152/1979 discloses an overlock sewing machine in which the movable claw is secured to a throat plate as in the previously mentioned prior art and is adapted to be moved by a conversion means which converts a rotary motion into linear motion, and a rotary dial is attached to this conversion means. This overlock sewing machine, however, still requires means for fixing the movable claw against the vibration of the cutter blade mechanism and, therefore, the user could not fully enjoy the advantage of the provision of the rotary dial. Needless to say, due to the fact that the movable claw is secured to the throat plate, this known overlock sewing machine encounters a difficulty in adopting an arrangement which would permit the movable claw to move between the operative position where it is in a side-by-side relation to the side of the stationary claw and the inoperative position where it is offset from the side of the stationary claw.
The known overlock sewing machine described hereinbefore suffers from a common disadvantage that, when the movable claw is spaced away from the stationary claw, the cloth undesirably drops into the gap between the stationary claw and the movable claw particularly when the cloth has a comparatively small thickness, so that it is quite difficult to obtain attractive and precise overlock.
Under these circumstances, the present invention aims as its primary object at providing an overlock sewing machine which can overcome the above-described problems of the prior art.
More specifically, it is an object of the invention to provide an easily operable overlock sewing machine which is improved to permit a change in the looping width merely by a rotation of a dial and to eliminate the necessity for the fixing means or the like for fixing the movable claw against the vibration of the cutter blade mechanism.
Another object of the invention is to provide an overlock sewing machine which is improved to avoid the undesirable dropping of the cloth into the gap between the stationary claw and the movable claw which may occur when the sewing operation is conducted with a large looping width, thereby to ensure an attractive appearance of the overlock even when the thickness of the cloth is comparatively small.
Still another object of the invention is to provide an overlock sewing machine which can attain, in addition to the objects mentioned above, an additional function of wrap-lock sewing.